The Chemistry of the Rocky Mountains

Introduction

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The Rocky Mountain Range is the largest mountain range in North America, and has been populated by humans for 12,000 years. They have seen indigenous tribes such as Crow, Sioux, Apache, Blackfeet, and Flathead; European explorers such as Lewis and Clark and Sir Alexander MacKenzie; Railroads and mountain men looking for furs and gold; and now millions of tourists visiting the many national parks, ski resorts, and trails that the Rocky Mountains have to offer. I chose the Rocky Mountains because my family spends a lot of time outdoors hiking and cross country skiing in the Rocky Mountains. I was interested to learn the chemistry and geology of what is under my feet!

Composition of ...

Granite

70-77% Silica (SiO₂)

11-13% Alumina (Al₂O₃)

3-5% Potassium oxide (K₂0)

3-5% Sodium bicarbonate (NaHCO₃)

2-3% Iron (Fe)

1% Calcium oxide (CaO)

Magma

Mafic or Basaltic

45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na

Intermediate or Andesitic

55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K

Felsic or Rhyolitic

65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na

Main Chemicals, Compounds, Components

Granite

Granite is an intrusive igneous rock, meaning it was formed (crystallized) under the earth's surface. It is formed at convergent plate boundaries. The Rocky Mountains were the result of two plates converging, creating granite predominantly, along with other igneous rocks.

Metamorphic Rock

The primary rock in the Rocky Mountains is metamorphic rock. Metamorphic rock happens when rocks are subjected to stress, high temperatures, pressure, and hydrothermal fluids (more about this in Chemistry's role). The two most common types of metamorphic rock in the Rockies are Schist and Gneiss. Schist is the metamorphosed form of shale, a sedimentary rock. Schist usually occurs when two plates converge on the continental side, so it makes sense it would be found in the Rocky Mountains. For a rock to be schist it does not have a certain mineral composition, it just has to be foliated and able to break into thin slabs in one direction. Gneiss is an even more metamorphosed version of schist that is granular.

Chemistry's Role

Granite is an igneous rock formed by felsic or rhyolitic magma below the earth slowly crystallizing. Felsic magma is 65-74% SiO₂, which accounts for the high percentage of Silica in granite. It has a high viscosity due to the large amount of SiO₂ and in turn, has a low temperature in comparison to other magma types, only 650-800 ℃ compared to Mafic magma, which is 1000-1200 ℃. Felsic magma also tends to be low in metal cations, more covalent, and have a low crystallization temperature. These factors account for its ability to crystallize under the earth's surface and become igneous rock, namely granite. Granite also varies based on how it crystallizes. Each mineral in magma crystallizes at a different temperature. Some crystals when they form are denser or lighter than the liquid they formed in, causing them to sink or float. This process, called fractional crystallization, changes the composition of the remaining liquid and can create variations in magma which make variations in rock. Granite varies from single- alkali-feldspar granite (formed in magma with less water) to two-feldspar granite (from magma with more water). Most of the granite found in the Rockies is in Rocky Mountain National Park, and it was formed during the Precambrian era (all of time before 600 million years ago).

Metamorphic rock is formed using a much different process. There are several factors that have to be present when a rock is metamorphosed; pressure, temperature, stress, and fluid. In order for a rock to change types, it has to change the minerals that it is composed of. Chemical reactions occur to change the minerals in a rock to minerals more suited for the new temperatures and pressures. As the depth of a rock increases, the pressure increases and the temperature varies. Pressure is a type of stress where the force is equally on all sides of an object, in this case a rock. Stress in which force is not equally distributed is called differentiable stress. Differentiable stress comes in many different types that all effect metamorphism. Stress flattens mineral grains in the direction of maximum stress and these grains create sheets of silicates called foliation. The schist rock found in the Rocky Mountains is an excellent example of flat grains and foliation in metamorphic rock. Fluid can then enter any open space between the mineral grains. Most of the fluid is H₂0, but it is filled with dissolved ions. The ions act as catalysts in the chemical reactions that change one form of rocks to another. The “ionic fluid” crystallizes in the rock, making new minerals. The crystals can also undergo polymorphism, where a solid can have more than one crystal form (an example of this is diamonds and graphite, which are two different crystalline forms of carbon). This creates variation in minerals that makes metamorphic rock in the Rockies so diverse.

Background Research

The Rocky Mountains, a mountain range known for the highest peaks in North America, stretches from British Columbia to New Mexico (3,000 miles). They were formed 80-55 million years ago (during the Jurassic to Cenozoic periods) by ocean plates subducting under continental plates and a low angle, pushing up mountains inland. The Rocky Mountains are mostly made of metamorphic rocks, specifically precambrian metamorphic rocks that were pushed up from under the crust as the mountains were formed. Metamorphic rocks change due to rising temperatures and pressures that change the chemical composition of the original rock. Fluid between rock layers that is mostly water but contains some dissolved ions also becomes a catalyst for changing the chemical composition of a rock. In addition to metamorphic rocks such as schist and gneiss, the Rocky Mountains, particularly the central region, have large igneous granite formations.

Ana Strong Garcia is a Junior at Billings Senior High School. She plays violin in the Billings Symphony, runs cross country, is a Montana Legislative Page, and participates in STEM club, Montana Code Girls, Model UN, and the Platinum program. When she is not doing her chemistry homework, Ana enjoys traveling, violin, hanging out with friends, and being outside.

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